This invention relates to control circuits for power converters and more specifically to power converters having an overload protection and voltage-control capability.
For power converters, it is usually necessary to provide a control circuit for short circuit protection and for regulating the output voltage. Typically, this may be accomplished by measuring output current and interrupting the converter switch when the amplitude of current is at a certain point. Alternatively the input current flowing through the transistor switch or the primary of the converter may be measured and the transistor switch turned OFF when the input current reaches a certain point.
Major disadvantages of prior art control circuits include considerable bulk and power dissipation when high currents are measured. Furthermore, in case of low output power converters, the area occupied by the control circuit and its costs become considerably more burdensome than for high power ones.
A disadvantage with measuring current is also experienced when the converter has to operate over a wide range of power ratings. For different power ratings, different power sensing designs become necessary resulting in higher cost and less flexibility.
A further drawback is that for certain converter topologies there is little correlation between the value of the transistor switch current and the value of the output current. The class of converters wherein the measurement of switch current cannot be easily used for control of output current includes cascaded power converters, known as "quadratic" converters such as buck-boost-buck, buck-boost/buck-boost and other derivations of the "quadratic" converter class.
Therefore, it would be desirable to provide short circuit protection without having to sense currents.